Polyolefins such as high density polyethylene (HDPE) homopolymer and linear low density polyethylene (LLDPE) copolymer can be produced using various combinations of catalyst systems and polymerization processes. In many olefin polymerization processes, a catalyst system containing more than one transition metal compound is utilized. Precise determination of the relative and absolute concentrations of each transition metal compound allows for better control of the polymerization processes and the resulting polymer products. It would be beneficial if real-time monitoring or measurement of the respective amount of each transition metal compound present in catalyst feed streams, catalyst systems, and polymerization reactor systems could be performed in order to improve the control of the polymerization process. Additionally, it would be beneficial to determine the concentration of a first transition metal compound in solutions where the UV-Vis spectrum overlaps with that of a second transition metal compound, and/or where a second transition metal compound is in large excess relative to the first transition metal compound. Accordingly, it is to these ends that the present invention is generally directed.